Steganografi dan Watermarking
Rinaldi Munir – IF5054 Kriptografi
7
• Sayangnya, metode modifikasi LSB kurang bagus untuk
steganografi, karena robustness-nya rendah. Selain itu, dapat terjadi kasus penurunan jumlah warna fidelity rendah.
dilakukan
7.5 Ukuran Data Yang Disembunyikan
• Ukuran data yang akan disembunyikan bergantung pada
ukuran citra penampung. Pada citra 24-bit yang berukuran 256
× 256 pixel terdapat 65536 pixel, setiap pixel berukuran 3
byte komponen RGB, berarti seluruhnya ada 65536 ×
3 = 196608 byte. Karena setiap byte hanya bisa menyembunyikan
satu bit di LSB-nya, maka ukuran data yang akan disembunyikan di dalam citra maksimum
1966088 = 24576 byte Ukuran data ini harus dikurangi dengan panjang nama berkas,
karena penyembunyian data rahasia tidak hanya menyembunyikan isi data tersebut, tetapi juga nama
berkasnya.
• Semakin besar data disembunyikan di dalam citra, semakin
besar pula kemungkinan data tersebut rusak akibat manipulasi pada citra penampung.
• Untuk memperkuat keamanan, data yang akan
disembunyikan dapat dienkripsi terlebih dahulu. Sedangkan untuk memperkecil ukuran data, data dimampatkan sebelum
disembunyikan. Bahkan, pemampatan dan enkripsi dapat juga dikombinasikan sebelum melakukan penyembunyian
data.
8
7.6 Teknik Ektsraksi Data
• Data yang disembunyikan di dalam citra dapat dibaca
kembali dengan cara pengungkapan reveal atau extraction. Posisi byte yang menyimpan bit data dapat diketahui dari
bilangan acak yang dibangkitkan. Bilangan acak yang dihasilkan harus sama dengan bilangan acak yang dipakai
pada waktu penyembunyian data. Dengan demikian, bit-bit data rahasia yang bertaburan di dalam citra dapat
dikumpulkan kembali.
•
Contoh program steganografi adalah DATAhide dari
program Tugas Akhir Lazarus Poli, NIM : 13593601, Penerapan Steganografi dengan Citra Dijital Sebagai File
Penampung, Tugas Akhir Teknik Informatika, 1998.
Untuk setiap contoh, digunakan kunci enkripsi yang sama: informatika
Upa-menu pada Operasi: Penyembunyian data dan Pengungkapan data
.
Steganografi dan Watermarking
Rinaldi Munir – IF5054 Kriptografi
9
Citra 24 bit
Penampung: citra peppers.bmp 512 ×
512 pixel, 769 KB
Data yang disembunyikan: citra handshak.bmp 44 KB
Hasil penyembunyian data peppers.bmp + handshake.bmp:
10
Hasil ekstraksi data:
Berkas handshak-stega.bmp hasil ekstraksi:
Steganografi dan Watermarking
Rinaldi Munir – IF5054 Kriptografi
11
Citra penampung: pepper.bmp 512 ×
512 pixel, lihat Gambar 7.1.
Data yang disembunyikan: citra hendro.doc 20 KB
LETTER OF RECOMMENDATION To Whom It May Concern,
Herewith I highly recommend Mr. R. Hendro Wicaksono continue his postgraduate study at your university. My recommendation is based on my experience as his lecturer in
several courses for the past four years. He has shown me his excellent attitude and personality. He is a hard working person and
he has a lot of creative ideas. He is also a very intelligent student and he cooperates very well with his peers whenever they had to work together.
During his study, he showed diligence and eagerness to achieve his goal. He sets ver high standard for himself and organizes himself very well to achieve the standard. I am
confident that if he can maintain his goal work, he should be able to complete the postgraduate program well within the stipulated time.
I am sure that his abilities and his personal qualities along with his academic capabilities will help his to obtain his Master’s degree at your university, which will be very useful
for our country. Bandung, November 15, 2002
Yours Sincerely, Ir. Rinaldi Munir, M.Sc.
Senior Lecturer Informatics Engineering Department,
Institute Technology of Bandung ITB Jl. Ganesha No. 10, Bandung 40132
Email : rinaldiinformatika.org Phone +62-22-2508135
Indonesia
12
Hasil penyembunyian data peppers.bmp + hendro.doc
Hasil ekstraksi data:
Steganografi dan Watermarking
Rinaldi Munir – IF5054 Kriptografi
13
Berkas hendro-stega.doc hasil ekstraksi:
LETTER OF RECOMMENDATION
To Whom It May Concern, Herewith I highly recommend Mr. R. Hendro Wicaksono continue his postgraduate
study at your university. My recommendation is based on my experience as his lecturer in several courses for the past four years.
He has shown me his excellent attitude and personality. He is a hard working person and he has a lot of creative ideas. He is also a very intelligent student and he cooperates very
well with his peers whenever they had to work together. During his study, he showed diligence and eagerness to achieve his goal. He sets very
high standard for himself and organizes himself very well to achieve the standard. I am confident that if he can maintain his goal work, he should be able to complete the
postgraduate program well within the stipulated time. I am sure that his abilities and his personal qualities along with his academic capabilities
will help his to obtain his Master’s degree at your university, which will be very useful for our country.
Bandung, November 15, 2002 Yours Sincerely,
Ir. Rinaldi Munir, M.Sc. Senior Lecturer
Informatics Engineering Department, Institute Technology of Bandung ITB
Jl. Ganesha No. 10, Bandung 40132 Email : rinaldiinformatika.org
Phone +62-22-2508135 Indonesia
14
Citra 8 bit
Penampung: citra barbara.bmp 512 ×
512 pixel, 258 KB
Data yang disembunyikan: chord.wav 95 KB – berkas musik dari Windows.
Steganografi dan Watermarking
Rinaldi Munir – IF5054 Kriptografi
15
Hasil penyembunyian data barbara.bmp + chord.wav
Terjadi penurunan kualitas gambar karena pengaruh penurunan jumlah warna color quantization
Hasil ektraksi data:
16
7.7 Watermarking
